1. Field of the Invention
The present invention relates to an apparatus for removing fluid from a fibrous web. More particularly, the present invention relates to a heated extended nip press in which the web is subjected for an extended period to increase pressure and temperature.
2. Information Disclosure Statement
In the manufacture of a paper web from stock, stock is ejected from a headbox onto a forming wire where the stock is initially dewatered to form a fibrous web. The fibrous web is subsequently pressed between cooperating rolls to remove excess water therefrom. Subsequently, the pressed web is guided around a plurality of heated dryer drums for drying the web.
The drying section of a papermaking machine of necessity requires considerable expenditure in the terms of thermal input in order to complete the drying process. Consequently, it has long been appreciated by those skilled in the art that the more water that can be pressed from the web in the press section, the less heat is required to remove remaining water in the drying section.
Extended nip presses have met with much success in increasing the water removing capability in the press section. Essentially, the extended nip press includes a backing roll and a cooperating elongate shoe with a bearing blanket extending through the extended nip defined between the backing roll and the shoe. The blanket moves through the extended nip and supports the web therethrough such that the dwell time of the web during passage through the extended nip is increased. Such increased residence time allows more water to be pressed from the web when compared with the more conventional cooperating roll press.
More recently, and as taught in U.S. Pat. No. 4,738,752 assigned to Beloit Corporation, the application of heat to an extended nip press causes the evolution of water vapor that assists in driving off even greater quantities of water in the liquid phase from the extended nip, thereby removing more water from the web compared with the more conventional extended nip press section.
Nevertheless, although in the aforementioned U.S. Pat. No. 4,738,752, the backing roll included a porous surface for the reception of water expelled from the web, the provision of such porous surface has included certain disadvantages.
More particularly, it is advantageous that the web in certain applications run in physical contact with the outer surface of the backing roll. Therefore, the porous surface of the backing roll should be provided with pore sizes of a size sufficiently small that marking of the web is negligible.
However, although various proposals have been set forth in co-pending patent application No. 07/089,887, assigned to Beloit Corporation, specifically setting forth various pore sizes, there exists a tendency for such porous surfaces to disintegrate under extended use of the heated extended nip press.
If the backing roll is not vented, the release of the internal pressure of steam from the paper when the web emerges from the extended nip press disrupts and damages the sheet structure and causes delamination thereof.
Although porous sintered metal surfaces have been used successfully on a laboratory scale in order to avoid the problem of delamination, the production of a high temperature porous backing roll for commercial application has proved difficult. The manufacture of a porous backing roll has been difficult particularly in view of the following problem. The sintered metal coating is composed of tiny particles with very small inter-particle bonding areas. When a relatively small stress is applied to a piece of sintered metal, much larger stresses occur at these bonding regions. Such larger stresses result in poor mechanical properties of the structure. Additionally, large sintered metal parts, such as backing rolls, are difficult to manufacture using current equipment and technology.
The present invention seeks to overcome the aforementioned problem by providing a plurality of very small grooves, or channels, along the surface of the backing roll. Such grooves allow the escape of fluid from the paper being dried. The aforementioned grooves provide sufficiently large inter-channel regions in order to give good mechanical integrity to the surface of the roll.
Therefore, it is a primary object of the present invention to provide an apparatus for removing fluid from a fibrous web that overcomes the aforementioned inadequacies of the prior art devices and which makes a considerable contribution to the art of high temperature extended nip pressing.
Another object of the present invention is the provision of an apparatus for removing fluid from a fibrous web in which the press member or backing roll has an outer surface which cooperates with the web and defines a plurality of grooves having a width within the range 1 to 1,000 microns for the reception therein of steam and water expelled from the web during passage of the web through the pressing section.
Another object of the present invention is the provision of an apparatus for removing fluid from a fibrous web in which each of the grooves has a depth within the range 5 to 600 microns and wherein each groove is spaced at a distance within the range 200 to 300 microns relative to an adjacent groove, the grooves extending circumferentially around a press roll.
Another object of the present invention is the provision of an apparatus for removing fluid from a fibrous web, the apparatus including a belt that extends through the pressing section. The belt surface defines a plurality of grooves which extend in a machine direction, such grooves having a width within the range 1 to 1,000 microns for the reception therein of steam and water expelled from the web during passage of the web through the pressing section.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter taken in conjunction with the annexed drawings.